1. Field of the Invention
The present invention relates to an ESD protection structure to protect a circuit element and the like from static electricity and a method for manufacturing the same.
2. Description of the Related Art
In general, various types of electronic equipment may be vulnerable to electrostatic discharge, e.g., a surge. In that case, an ESD (Electro-Static-Discharge) protection structure may be adopted to protect a circuit element from electrostatic discharge. An ESD protection structure is a structure that leads an excessive voltage from a signal line to a ground line by utilizing a discharge phenomenon in order to protect the circuit element from electrostatic electricity.
At present, a device, in which a hollow portion is disposed in a glass ceramic substrate, a pair of discharge electrodes connected to the signal line and the ground line, respectively, are disposed opposite to each other in the hollow portion, and particulate discharge-assisting electrodes are disposed dispersedly between the pair of discharge electrodes, has been developed as a device having the ESD protection structure (refer to, for example, International Publication No. 2010/061550). In this device structure, in the case where an excessive voltage is applied to the signal line, discharge occurs between the pair of discharge electrodes through discharge-assisting electrodes, so that static electricity is led to the ground.
Here, the mechanism that causes an occurrence of a discharge phenomenon in this device configuration will be described with reference to FIGS. 1A and 1B. FIGS. 1A and 1B are diagrams showing a configuration example of an ESD protection device including an ESD protection structure.
An ESD protection device 100A shown in FIG. 1A includes an insulating substrate 102, a portion of the region of which is exposed at the inside of a hollow portion 101, discharge electrodes 103A and 103B disposed while being stacked on the insulating substrate 102 in such a way that end portions exposed at the inside of the hollow portion 101 are opposed to each other, and a plurality of discharge-assisting electrodes 104 disposed dispersedly between the discharge electrodes 103A and 103B. The discharge-assisting electrodes 104 are made from insulating material-coated metal particles. Meanwhile, an ESD protection device 100B shown in FIG. 1B is an example in which a hollow portion 101 is formed not into a square shape, but into a hemispherical shape.
When a high voltage is applied between the discharge electrodes 103A and 103B, electrons are released from one electrode 103A. The electrons induce a secondary electron avalanche phenomenon due to collision with atoms in a high electric field and, thereby, many electrons are released from the atoms, so that discharge occurs between the discharge electrodes 103A and 103B. This discharge occurs as creepage discharge on a creepage surface between the hollow portion 101 and the insulating substrate 102. The discharge-assisting electrodes 104 made from insulating material-coated metal particles are disposed dispersedly between the discharge electrodes 103A and 103B, and a larger number of creepage surfaces are formed, so that the ESD discharge responsivity becomes stable.
Variations in interval of disposition of the discharge-assisting electrodes cannot be suppressed, and the ESD discharge responsivity cannot be stabilized completely, so that variations occur between products. Specifically, discharge occurs at a low voltage easily in a place where the particle interval between adjacent discharge-assisting electrodes is small and a place where particles are in contact with each other. This causes a reduction in breakdown voltage in the ESD discharge responsivity.
In addition, variations in the ESD discharge responsivity also occur because of coverage of circumferences of the discharge-assisting electrodes with a vitreous substance in production. Specifically, various constituent materials around the hollow portion are altered to become an insulating vitreous substance because of an oxidation reaction in the production (in firing or the like). Discharge through the discharge-assisting electrodes is prevented by coverage with this vitreous substance. This causes an increase in breakdown voltage in the ESD discharge responsivity, in contrast to the above-described variations in interval of disposition.
Furthermore, because of the above-described factors, not only variations between products, but also fluctuations due to repeated discharge occur in the ESD discharge responsivity. Specifically, reactions, e.g., vaporization, due to discharge occur at a location at which discharge phenomena concentrate, a change in a fine structure is induced, and this causes fluctuations in the ESD discharge responsivity due to repeated discharge.